Internal fault currents in A-C motors

VARIOUS TYPES OF PROTECTIVE DEVICES, such as relays, fuses, and circuit breakers, are used to provide a-c-motor fault protection. The effectiveness of these devices in protecting motors is, in most cases, dependent on the magnitude of the fault current. A general answer to this question of the magnitude of fault currents under different conditions of a-c system grounding, system capacity, fault location, and motor connections is presented. Results are approximate, but the results provide a reasonable basis for the comparison of the effectiveness of different methods of short-circuit protection.